1. Field of the Invention
The present invention relates to a surface acoustic wave element.
2. Background of the Related Art
A Surface Acoustic Wave (SAW) element is mounted on an antenna sharing device of a communication device, such as a portable terminal or the like, to pass a signal of a specific frequency band. A surface acoustic wave element of a longitudinal mode combination type configured of a plurality of comb-shaped electrodes (IDT: Inter Digital Transducers) is known as a surface acoustic wave element.
Generally, a portable terminal or the like adopts a communication method in which a frequency band of a reception Rx signal (hereinafter, referred to as a reception band) communicates in a high frequency band, compared with a frequency band of a transmission Tx signal (hereinafter, referred to as a transmission band). However, recently, a communication method in which the reception band is adjacent to the transmission band and, in addition, the transmission band communicates through a signal of a frequency higher than that of the reception band is also appeared.
As described above, in order to realize stable communication while the reception band is higher than and adjacent to the transmission band, a characteristic of steep and large attenuation at the high band side of the reception band is required in a surface acoustic wave element. In addition, when a communication using two or more bands is performed using one common antenna, it is regarded that a characteristic of steeply and highly attenuating a signal even in an attenuation band of the low band side, as well as the high band side, of the reception band will be required.
Patent documents 1 and 2 disclose a surface acoustic wave filter having a steep cutoff characteristic of low loss. Such a surface acoustic wave filter is a Double Mode SAW (DMS) filter and realizes the steep cutoff characteristic of low loss by withdrawing an input terminal and an output terminal in the same direction. In addition, they also describe that the cutoff characteristic of the surface acoustic wave filter can be improved by decreasing the opening of the IDT.
(Patent documents 1) Gazette of Japanese Laid-opened Patent No. 2004-194269
(Patent documents 2) Gazette of Japanese Laid-opened Patent No. 2006-42398